Automatic adjustment and compensation of the secondary voltage of a transformer



June 15, 1965 M. BROZEK 3,189,315

AUTOMATIC ADJUSTMENT AND COMPENSATION OF THE SECONDARY VOLTAGE OF ATRANSFORMER Flled May 3, 1961 2 Sheets-Sheet 1 F 16. la

v are) I 4m 8 & a Q A 2 3 a a s s. s 'ji ig i FIG 4 r mar BROZEKAUTOMATIC ADJUSTMEIiT AND COMPENSATION OF THE June 15, 1965 M BROZEK3,189,816

SECONDARY VOLTAGE OF A TRANSFORMER Filed May 3, 1961 2 Sheets-Sheet 2 OJX J; 3 o min./(V 3 O A T TORNE Y.

United States Patent 3,189,816 AUTOMATIC ADJUSTMENT AND CQMPENSATL'GN OFTHE SEQGNDARY VGLTAGE @F A TRANS- FORMER Mnata Breach, Modrany, nearPrague, Czechoslovakia, assignor to Chirana Praha, narodni podnik,Prague, Czechoslovakia Filed May 3, 1961, Ser. No. 107,558 Claimspriority, application Czechosiovakia, May 6, Edit, 2,988/69 6 tllaims.(Cl. 323-435) This invention relates to an improved method for automaticadjustment and compensation of the secondary voltage of a transformer,the method being based on the change of the number of turns of theprimary winding.

There exists a great number of apparatuses in which the Working voltagehas to be changed within a certain range. For example in the case oflarge X-ray instruments the values of actually used high voltages varyfrom 40 to 150 kilovolts (peak values). Changes in working voltage arealso necessary, in the case of electric filters, electrostatic sortingequipments, supply sources and in many other instances.

There are three generally known methods for changing the value of thesecondary voltage. In the first method, an auto transformer is connectedacross the primary winding. In the second arrangement, the number ofturns of the secondary winding is varied. In the third arrangement, thenumber of turns of the primary winding is varied.

These known arrangements have various disadvantages. In the case of thefirst arrangement mentioned above, a separate high powerauto-transformer must be built into the apparatus, and this involvesadditional expense. Variations of the number of turns of the secondarywinding, while eliminating the necessity for the use of the extraautotransformer, involves direct variation of the relatively very highsecondary voltage. In turn, this introduces substantial problems withrespect to insulation and to possible arcing and the like. The thirdarrangement, which is that of varying the number of turns of the primarywinding, does not have the disadvantages of the first two arrangementsmentioned above.

Neverthless, this third arrangement has not been used up to the presentfor various reasons. One reason is that it has not been known how tocontrol and stabilize the secondary voltage in such a way that thedevice could operate fully automatically and with a great degree ofaccuracy within the whole working range. An other difficulty of thisthird arrangement resides in the fact that the potentiometer in theautomatic controller must have a hyperbolic characteristic and that thesensitivity of the control relay for the servo-system must be aquadratic function of the difference between the maximum and minimumvalue of the voltage to be regulated.

These (lllllClllllfiS with respect to the third-mentioned controlarrangement have prevented such arrangement from being utilized inpractical operation.

The method of voltage control, or the secondary voltage control system,to which this invention relates eliminates the afore-mentioned drawbacksin that it suggests an accurately working practicable device based onthe third principle mentioned above. A substantial advantage of theinvention system consists in that the sensing element need not becalibrated as to its sensitivity since the sensitivity of the sensingelement can be constant, thus enabling the system to incorporateelectronic components and circuitry. For an understanding of theprinciples of the present invention, reference is made to the followingdescription of a typical embodiment thereof as illus- 3,189,816 FatentedJune 15, 1%65 trated in the accompanying drawings. In the drawings:

FIGS. la, 1b and 1c are schematic wiring diagrams illustrating the threeknown secondary voltage control systems mentioned above;

FIG. 2 is a somewhat more detailed schematic Wiring diagram of a controlsystem based uponthe principle .of controlling the secondary voltage byvarying the number of turns of the primary .winding;

FIG. 3 is a somewhat more detailed schematic wiring diagram illustratinga secondary voltage control system embodying the invention; and

FIG. 4 is a graph illustrating the principles involved in the inventioncontrol system.

F IG. la illustrates a system for controlling the secondary voltage of atransformer Tm by the use of an auto transformer AT which is connectedacross the primary winding of the transformer. KV represents a meter,and the load, such as an X-ray tube, as indicated as L.

PEG. lb illustrates an arrangement for controlling the secondaryvoltageof the transformer Tm by varying the number of turns of thesecondary winding, this being effected by the use of a moveable contactMC, as schematically illustrated in FIG. lb.

FIG. 1c schematically illustrates a secondary voltage control systemwhich is based upon the principle of varying the number of turns of theprimary winding. In this arrangement, upon which the present inventionis an improvement, the number of turns of the primary winding is variedby means of an adjustable contact MC.

The basic transformer equation suggests that for a-constant number ofsecondary turns the number of primary turns can be computed for eachchosen value of both the main voltage and the secondary voltage.

The basic transformer equation is diagrammatically illustrated in thegraph of FIG. 4 covering the usual working ranges. At the same time,this graph also shows the nomograph of the equation expressing theregulating action of the device according to this invention.

Prior to go more deeply into the description of circuits shown in FlGS.2 and 3 yet us explain the meaning of abbreviations and symbols to befound in these figures:

Referring to FIG. 2, the potentiometer moving contact I is associatedwith the scale of the control device. The corresponding potentiometer Pis linear and a stabilized DC. voltage is applied to its terminals. Thecontrol moving contacts 3 which adjusts the number of primary turns, is.coupled to the moving contact J of the hyperbolic potentiometer P Thispotentiometer is connected across a DC. voltage which is proportional tothe voltage of that phase of the mains x, y, z, which is applied acrossthe main transformer TR. Both moving contacts 1 and J are electricallyconnected through the coil of a polarized relay PR. When this relay isnot excited its moving contact is in the intermediate (zero) positionbetween two fixed contacts. This condition exists when the secondaryvoltage is at the proper value corrresponding to the setting of thecontact J with respect to the scale associated therewith. This relay(sensing element, pick-up) can be either electromagnetic (moving-coilsystem) or electronic or it can be based on a semi-conductor etc. Therelay contact is in neutral position when the voltages picked-up by bothcontacts 1 and 1 are equal. lt-mr instance-the voltage on I is higherthan that on J as when the secondary voltage is lower than a desiredpreset value, the relay contact closes the circuit of the relay n which,in turn, energizes the electro-motor M to rotate in such a directionthat the voltage tapped with the contact J begins to rise. As soon asthe voltages on 3 and 1 are equal, the contact of the relay PR returnsto neutral position, the relay n is de-energized, the electro-motorceases to rotate and the whole device is again at rest.

Table showing the mains voltage compensation and adjustment ofkilovolts:

Z1 {or Zr for Z1 for Sensi Z1 32% Kv. KV.rma 320 v. 380 v. 420 v. Vxegtivity, 420 v.

28. 3 640 760 840 80 0. 4 200 35. 4 510 605 670 100 0. 625 160 42. 5 422500 554 120 0. 91 132 49. 5 363 430 476 140 1.24 113 5G. 5 320 380 420160 l. 6 100 63. 5 288 342 378 180 2 90 70. 6 259 307 340 200 2. 47 8177. 8 234 278 307 220 3. 02 73 84. G 213 252 279 240 3. 64 66 92 196 233257 260 4. 6!. 99 182 216 239 280 4. 9 51 106 170 202 223 300 5. 67 53113 160 190 210 320 G. 4

In the foregoing table Z means the number of primary turns.

The full sensitivity range is 0.4:6.4=1:16.

It is apparent from the above example that, in existing arrangements forvarying the secondary voltage by varying the number of turns of theprimary winding, of which FIG. 2 illustrates a typical arrangement, thepick-up sensitivity must be changed in the ratio 1:16 if the assumedcontrol range is to be covered. In case of a moving-coil relay this canbe achieved with the aid of a variable resistor P connected in parallelto the relay coil while the moving contact 1.; of this resistor ismechanically linked to 5 the moving contact 1 Let us now discuss theproblem of the aforementioned voltage control more thoroughly:

The basic transformer equation can be written in the following simpleform:

E 2 E2 with Z Z meaning the number of primary and secondary turns and EE being the primary and secondary voltages. Since in X-ray equipment itis usual to consider the peak voltage, the above equation can bere-written as follows where Z =const.=56,500 turns (for E/Z=2) E variesfrom 40- kv to 160 kv E varies from 320 v. to 420 v.

Z varies from 160 to 840 turns Keeping in mind that Z =const. the formerequation can be transformed into:

Substituting 13 and 2 :2. we may draw a graph, as shown in FIG. 4 withthree systems of straight lines where Z are straight lines parallel toY-axis, E are parallel to X-axis and E form a cluster of straight lines,all passing through the origin and having slopes equal to ZZ /Z Thegraph of PEG. 4 shows how many turns of the primary winding are neededfor the full range of control for each secondary voltage and for thecorresponding change of mains voltage varying rorn 320 to 420 volts.

It is also possible to introduce a new variable E (i.e. control volts)into the aforementioned graph. Obviously, this new variable should besubstituted for Z using the relation Z.const=E,. with the constant equalto the control voltage for Z21. Thus, a constant (C) must be introducedinto this equation so as to produce E =CZ Now, we have to substituteE,.:const.Z :CZ into the equation and-in order to secure E,.:const.Z thevoltage across the control potentiometer P must be stabilized.

Since Czconst. and Z =const., the substitution leads to the basicequation for the control voltage:

With the sensitivity for the change equal to one turn or one step K(e.g. K=5 turns) being constant, the pick-up sensitivity need not bevaried at all. The control voltage E is directly proportional to thepick-up sensitivity and inversely proportional to the secondary voltage.

The equation defining the mains voltage compensation for a constanthigh-voltage can be written in the form and this is an analytic equationof a straight line.

The equation determining the high-voltage adjustment for constant mainsvoltage reads K zan,

and this is the well known equation of the hyperbola.

From the last quoted relation, the kilovolt scale can be computed, e.g.for the nominal mains voltage E :38() volts.

This equation also enables the extreme value B to be computed. Thus,another scale can be added to the Z- scale in the graph of FIG. 4mentioned above. For instance, for 1K:5 turns and for the pick-upsensitivity C:2 volts per one turn, the required control volts can beread from the diagram at a glance.

The actual control circuit based on the above deduced mathematicalanalysis, is diagrammatically shown in FIG. 3.

Referring to FIG. 3, the control potentiometer P is a linearpotentiometer and has applied thereacross a stabilized DC voltage ratherthan a DC. voltage proportional to the voltage of that phase of themains which is applied across the transformer 1",. It will thus be notedthat the arrangement of FIG. 3 ditters, in this first respect, from thatof FIG. 2. The potentiometer P of the invention arrangement, as shown inFIG. 3, has applied thereacross an unstabilized voltage which is a DC.voltage proportional to the voltage of that phase of the mains x, y, z,which is applied across the primary winding of the transformer T Theconstant I, of the potentiometer P and the contact J of thepotentiometer P are interconnected electrically through the differentialsensing device such as the polarized relay PR. The potentiometer P canbe (1) a linear potentiometer with a hyperbolic scale, (2) a hyperbolicpotentiometer with a linear scale, or (.3) a combination ofpotentiometer and associated scale having a hyperbolic characteristic.It will be noted, from reference to the hyperbolic characteristicindicated at A, and from reference to the scale, that the voltageadiustment increases in a downward direction, across the potentiometer Pas compared to the potentiometer P of FIG. 2 wherein the voltageincreases in an upward direction. Thus, the arrangement of FIG. 3corresponds to the aforementioned mathematical analysis, wherein it waspointed out that the control voltage E, is inversely proportional to thesecondary voltage of the transformer TR, and wherein it was pointed outthat the equation determining the high voltage adjustment for a constantvoltage of the mains is an equation for a hyperbola.

While a specific embodiment of the invention has been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What I claim is:

1. in a control system for automatically controlling the secondaryvoltage of a transformer by correctively adjusting the number of primaryWinding turns of the transformer responsive to deviation of thesecondary voltage from a preset value as a result of variation in thesupply voltage impressed across the transformer primary winding, theimprovement comprising a first contact adjustable along said primaryWinding to vary the number of turns thereof effectively in circuit; afirst linear potentiometer including a second contact adjustabletherealong and mechanically coupled to said first contact; means forapplying a stabilized DC. voltage across said first potentiometer; scalemeans; second potentiometer means; a third contact adjustable along saidsecond potentiometer means with reference to said scale means to presetsaid voltage value; a servo-system for conjointly adjusting said firstand second contacts; means for impressing, across said secondpotentiometer means, a DC. voltage proportional to the AC. voltageimpressed across the primary winding of said transformer, to provide acontrol voltage; and differential responsive control means forcorrectively operating said servo-system responsive to a difference inthe adjusted DC. voltages represented by said second contact and saidthird contact; at least one of said second potentiometer means and saidscale means being hyperbolic.

2. In a control system improvement, as claimed in claim 1, said controlvoltage impressed across said second potentiometer means being inverselyproportional to the secondary voltage of said transformer.

3. In the control system improvement, as claimed in claim 1, thesensitivity of said differential responsive control element beingconstant over the complete regulating range of the system.

4. In a control system improvement, as claimed in claim 1, said secondpotentiometer means comprising a linear potentiometer and said scalemeans comprising a hyperbolic scale.

5. in a control system improvement, as claimed in claim 1, said secondpotentiometer means comprising a hyperbolic potentiometer and said scalemeans comprising a linear scale.

6. In a control system improvement, as claimed in laim l, thecombination of said second potentiometer means and said scale meanshaving a hyperbolic characteristic.

No references cited.

LLOYD MCCOLLUM, Primary Examiner.

MELTON O. HERSHFIELD, Examiner.

1. IN A CONTROL SYSTEM FOR AUTOMATICALLY CONTROLLING THE SECONDARYVOLTAGE OF A TRANSFORMER BY CORRECTIVELY ADJUSTING THE NUMBER OF PRIMARYWINDING TURNS OF THE TRANSFORMER RESPONSIVE TO DEVIATION OF THESECONDARY VOLTAGE FROM A PRESET VALUE AS A RESULT OF VARIATION IN THESUPPLY VOLTAGE IMPRESSED ACROSS THE TRANSFORMER PRIMARY WINDING, THEIMPROVEMENT COMPRISING A FIRST CONTACT ADJUSTABLE ALONG SAID PRIMARYWINDING TO VARY THE NUMBER OF TURNS THEREOF EFFECTIVELY IN CIRCUIT; AFIRST LINEAR POTENTIOMETER INCLUDING A SECOND CONTACT ADJUSTABLETHEREALONG AND MECHANICALLY COUPLED TO SAID FIRST CONTACT; MEANS FORAPPLYING A STABILIZED D.C. VOLTAGE ACROSS SAID FIRST POTENTIOMETER;SCALE MEANS; SECOND POTENTIOMETER MEANS; A THIRD CONTACT ADJUSTABLEALONG SAID SECOND POTENTIOMETER MEANS WITH REFERENCE TO SAID SCALE MEANSTO PRESET AND VOLTAGE VALUE; A SERVO-SYSTEM FOR CONJOINTLY ADJUSTINGSAID FIRST AND SECOND CONTACTS; MEANS FOR IMPRESSING, ACROSS SAID SECONDPOTENTIOMETER MEANS, A D.C VOLTAGE PROPORTIONAL TO THE A.C. VOLTAGEIMPRESSED ACROSS THE PRIMARY WINDING OF SAID TRANSFORMER, TO PROVIDE ACONTROL VOLTAGE; AND DIFFERENTIAL RESPONSIVE CONTROL MEANS FORCORRECTIVELY OPERATING SAID SERVO-SYSTEM RESPONSIVE TO A DIFFERENCE INTHE ADJUSTED D.C. VOLTAGES REPRESENTED BY SAID SECOND CONTACT AND SAIDTHIRD CONTACT; AT LEAST ONE OF SAID SECOND POTENTIOMETER MEANS AND SAIDSCALE MEANS BEING HYPERBOLIC.